Issue 30

G. Bolzon et alii, Frattura ed Integrità Strutturale, 30 (2014) 31-39; DOI: 10.3221/IGF-ESIS.30.05

Indentation provokes the detaching of scale crystals in the piling-up area of the residual imprint, evidenced by the micrographs in Fig. 6 and, in more detail, Fig. 7 and Fig. 8 (left). The effect of this phenomenon on the indentation curves is magnified by the axis-symmetric Rockwell tip although the areas of removed scales are comparable, see Fig. 6. In the region under the indenter tip, crystals are compact and no further damaging phenomena are observed. The situation is rather different on side B, exposed to the fluid, where a thick scale coating is deposited on a corroded layer containing cementite, see Fig. 2. The corresponding indentation curves are drawn in the graphs of Fig. 4 (right) and Fig. 5 (right). The trend is typical of fracture propagation in brittle materials [15]: a series of kinks appear on the loading branch starting from about 40÷50 N. Cracks are clearly observed in the region underneath the wide areas of removed carbonate crystals, shown in Fig. 8 (right) and Fig. 9. The separation between the two corrosion layers is also observed, even at far distance from the imprints, see Fig. 10.

Figure 6 : SEM micrograph (BSE) of the indented surface of X65 specimen, side A in contact with the autoclave bottom. Rockwell (left) and Vickers (right) tip at 200 N maximum load. Same magnification, marker: 1 mm.

Figure 7 : SEM micrograph (SE) of the specimen surface (left, marker: 200 µm ) and on the cross section (right, marker 50 µm) of X65 corroded sample after Rockwell indentation at 200 N maximum load: side A, in contact with the autoclave bottom.

Figure 8 : SEM micrographs of the specimen surface after Vickers indentation at 200 N maximum load of X65 corroded sample: side A in contact with the autoclave bottom (left, SE, marker: 200 µm ) and side B exposed to the fluid (right, BSE, marker 100 µm).

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